1. Field of the Invention
The present invention relates to an automatic temperature control system for a heated roller of a fixing device mounted in a copying machine, a microfilm printer or the like. More specifically, the present invention relates to an automatic temperature control method and an apparatus therefor which provide the supply of a control signal to control a heater for heating the heated roller, the on/off control signal being related to a comparative valve between a redetermined temperature and a sensed temperature.
2. Description of the Prior Art
Generally, a conventional temperature control system of this type effects controls in response to a signal provided by a comparator to switch off the heater when the surface temperature of the heated roller exceeds a predetermined temperature and switch on the heater when the surface temperature falls below the predetermined temperature.
In recent years, however, use of a small heated roller having a small heat capacity or a large capacity heater has been proposed in order to shorten the warming-up time. But this in turn not only causes increase a unit time temperature rise when the heater is switched on but also to cause sharp temperature drop with passage of copy paper. On the other hand, a temperature detecting device such as a thermister itself has some heat capacity which causes a time lag between its detection of a temperature change and its corresponding output. With the conventional temperature control system, such a time lag occurs in the because of rapid temperature changes, which widens the difference between a detected temperature and an actual temperature. As a result, temperature ripples with respect to the predetermined temperature become large. In that situation the conventional control system is not only incapable of high precision temperature control but has the drawback of damaging the heated roller with the enlarged temperature ripples.